This invention relates to wafer handling apparatus and, in particular, to an electrostatic clamp for holding semiconductor wafers.
As known in the art, clamps for semiconductor wafers enable one to improve heat transfer between a wafer and an electrode upon which it rests. Without a clamp of some sort, a gap exists between the wafer and the electrode except at a few points of contact, even for nominally flat wafers and electrodes. Heat transfer is very dependent upon the air (gas) pressure within this gap. At low pressure, such as used in processing the wafer, heat transfer is very poor.
Clamping a wafer increases the number of points of contact. If compliant material is used between the wafer and the electrode, the area of contact is increased and heat transfer is improved. Good heat transfer, whether to or from the wafer, enables better control of the temperature of the wafer, resulting in better process control.
A variety of electrostatic clamps have been proposed in the prior art. In U.S. Pat. No. 4,184,188, an electrostatic clamp is disclosed comprising parallel lines formed by interdigitated, printed circuit conductors. The lines are insulated from the wafer by RTV silicone and are formed on a thermally conductive, electrically insulative support.
In U.S. Pat. No. 4,480,284, the wafer clamp is disclosed as comprising a conductive material having dielectric material flame spray coated thereon and then smoothed with a layer of plastic. The semiconductor wafer forms one plate of a capacitor having the conductive material as the other plate.
In U.S. Pat. No. 4,502,094, the static voltage is also applied to the wafer which rests on a conductive support having a plurality of electrodes formed therein and separated from the support by insulating material.
While describing functional wafer clamps, some of these patents describe rather high voltage, e.g. 4,000 volts, as typical. In applications such as plasma etching, the high voltage can cause problems with uniformity of the etch. Further, one typically couples RF energy through the electrode on which the wafer rests. Electrostatic clamps of the prior art are not particularly suited to coupling RF power to a semiconductor wafer due to the dielectric films on the clamp. Finally, there is a continuing need to reduce the cost of manufacturing the wafer clamp.
In view of the foregoing, it is therefore an object of the present invention to provide an improved electrostatic wafer clamp.
Another object of the present invention is to provide an easily manufactured electrostatic wafer clamp.
A further object of the present invention is to provide an electrostatic wafer clamp for coupling RF power to a wafer.
Another object of the present invention is to provide an electrostatic wafer clamp having higher clamp strength for a given applied voltage.